The presently disclosed embodiments are generally directed to methods and systems for evaluating and preparing pigment dispersions meeting specific desired characteristics. More specifically, the present embodiments are directed to methods for evaluating particle size of colorless or light color dispersions using a novel parameter described as particle size related scattering index (PSRSI) to determine particle size and distribution characteristics.
Methods for evaluating the characteristics of pigment dispersions are important for determining whether the evaluated dispersion has the target qualifications for a specific application. The methods may be used to monitor the preparation of dispersions or emulsions for use in a variety of applications, for example, in manufacturing particle dispersions, creams and lotions, emulsified oils, foods, cosmetics, cleaning fluids and precursors to many industrial products, such as coatings for photoreceptor layers. The general process of making ink involves the grinding of pigment in a device, such as a mill, to form a pigment dispersion, and subsequently letting down the pigment dispersion with suitable resins to meet certain rheological and functional properties. Thus, pigment selection and proper dispersion are of great importance to cost efficiency of making such products.
The dispersion process involves the breakdown of particles into smaller particles and their even distribution in a fluid, leading to a colloidal suspension. Pigment dispersion generally involves a complete wetting of the pigment particles, a breakdown of pigment particles, and stabilization of the dispersion to maintain a colloidal system during further processing or storage of the dispersion. Stabilization prevents the finely dispersed particles from reagglomerating or flocculating. The breakdown of pigment particles may be performed during the wetting process or mechanically by transferring mechanical energy into the system through special dispersing equipment. In some cases, particle dispersion can be prepared starting from solution by chemical reaction such as precipitation reaction or by physical change such as precipitation via solvent change. Emulsion dispersion can be formed either by dispersing and emulsifying liquid mixture or by chemical reaction such as emulsion polymerization.
The quality of the final dispersion, including particle dispersion and emulsion, is dependent on the optimization of many influencing factors. One of these factors is the size of the pigment particle or droplet. To achieve the optimum benefits of a pigment or emulsion, it is desirable to obtain a dispersion which particle size is as close to the target as possible.
As such, there have been different methods devised to evaluate dispersions for ensuring that the achieved dispersions have attained the desired characteristics. For example, a relative scattering index (RSI) is a commonly used parameter for evaluating colored pigment dispersions, which is defined as the absorbance ratio between the absorbance at a specific wavelength where the absorbance mainly reflects the contribution of light scattering, and the absorbance at the absorption peak where the absorbance is mainly the contribution of absorption.
RSI has been proven to be a good parameter for dispersion characterization and qualification that can be conveniently and quickly determined by a spectrophotometer. However, such a method is not applicable for colorless or light color dispersions where there is no absorption peak or where, at the peak wavelength, the contribution of light scattering to the apparent absorbance is not negligible since the RSI calculation requires such a peak absorbance to normalize the scattering signal. In colorless or light color dispersions, the contribution of absorption to the absorbance at the measuring wavelength is negligible as compared with the contribution of scattering.
Therefore, there is a need for a new method that can be used to efficiently and accurately evaluate and prepare colorless or light color dispersions that meet desired overall particle size and distribution properties, and a system that implements that method.